Pressure-sensitive recording sheets may consist of a combination of an upper sheet comprising a microcapsule layer, which contains microcapsules in which oil droplets obtained by dissolving a more or less colorless electron-donating dye (referred to hereinafter as a color former) in an appropriate solvent are microencapsulated, coated on a support, a lower sheet comprising a developing layer, which contains an electron-accepting compound (referred to hereinafter as a color developer), coated on another support and, depending on the particular case, intermediate sheets which have a microcapsule layer coated on one side of a support and a color developer coated on the other side of a support, or in some cases the aforementioned capsules and color developer are contained on the same side of a support, and in some other cases either the capsules or the color developer aforementioned is included in the support and the other is provided as a coating.
Such pressure-sensitive recording sheets have been disclosed, for example, in U.S. Pat. Nos. 2,505,470, 2,505,489, 2,550,471, 2,730,457, and 3,418,250. Known methods for the manufacture of the microcapsules used in pressure-sensitive recording sheets include, for example, coacervation methods, surface polymerization methods and in-situ polymerization methods. Coacervation methods have been disclosed, for example, in U.S. Pat. Nos. 2,800,457, 2,800,458 and 3,687,865, surface polymerization methods have been disclosed, for example, in U.S. Pat. Nos. 3,429,827, 3,577,515 and 3,886,085, and in-situ polymerization methods have been disclosed, for example, in U.S. Pat. Nos. 3,726,804 and 3,796,669.
Among these methods, microencapsulation by in situ polymerization and surface polymerization using the reaction between a polyisocyanate and water, a polyamine or a polyhydric alcohol has achieved some importance in recent years.
Microcapsules obtained using poly(urethane urea) films are advantageous in that it is possible to provide capsule solutions which have a higher concentration than those can be obtained by microencapsulation by coacervation with gelatin as widely used in the past, in that the resulting capsules have excellent water resistance, and in that encapsulation can be achieved in a short period of time.
However, the known microcapsules obtained using poly(urethane urea) films suffer from the following disadvantages.
1) They have inadequate resistance to solvents.
The oil droplets inevitably leak out from the capsules when in contact with organic solvents or when placed in an organic solvent atmosphere.
2) They have inadequate pressure resistance.
The capsules are ruptured on the application of low pressures, such as those imposed during print cutting and finishing, and fogging occurs.
3) The color formers react and a coloration is formed when isocyanates are used.
Aromatic based poly-isocyanates such as polymethylenepolyphenyl isocyanate form dark colors with fluoran compounds which have an --NH-- group, such as 2-anilino-3-methyl-6-diethylaminofluoran, and this is undesirable in the case of microcapsules for use in pressure-sensitive recording sheets.